1. Field of the Invention
The present invention relates generally to dispersion compensation in optical fiber transmission systems, and particularly to a differential delay component for use in a polarization mode dispersion (PMD) compensator.
2. Technical Background
The polarization of transmitted light is an important factor affecting the signal quality and available bandwidth (or channel spacing) in single-mode (SM) optical fibers. While single-mode fibers are usually characterized as carrying or supporting only one mode, they actually carry two degenerate modes each having orthogonal polarization relative to one another. By degenerate, it is meant that a single-mode optical fiber having a circularly-symmetric core would not differentiate between the two polarization modes, which are functionally identical or indistinguishable for most purposes. Given a single-mode fiber core having perfect circular symmetry, the presence of two distinct polarization modes would have negligible impact on optical fiber communications. However, in practice these two modes may be subject to polarization mode dispersion (PMD), in which the two polarization modes encounter dissimilar physical conditions or optical properties within the fiber, and therefore travel at slightly different speeds relative to one another. The different transmission speeds cause the polarization modes to spread or separate, creating a delay or phase offset between the two modes which is analogous to chromatic and other types of dispersion. Polarization mode dispersion can significantly denigrate the integrity of certain high-performance optical communications systems, particularly time-division-multiplexed signals operating on the order of 2.5 Gbit/second or faster.
There are several recognized causes of polarization mode dispersion which can be grouped into two main classes: birefringence and mode coupling. The causes may also be intrinsic to the fiber, or extrinsically induced. For example, the core of conventional single-mode fiber is not perfectly symmetric about the longitudinal axis, and non-uniformities in the index of refraction may vary both radially and axially over distances. These non-uniformities may result from deviations in normal dopant concentrations, physical stresses induced when the fiber is drawn or wound on a spool, or external physical pressures (sometimes called xe2x80x9clateral loadingxe2x80x9d) induced by operations such as coating and cabling the optical fiber. Mode coupling (or xe2x80x9cenergy transferxe2x80x9d) may result from coupling sites within the glass itself, fiber-to-fiber contact, contact between a coated optical fiber and other objects, or the bends and twists introduced into the length of optical fiber as it is laid, routed, spliced, or connected.
Longer lengths of optical fiber and complex optical transmission networks can then be viewed as a multiplicity of concatenated birefringent segments, with the output optical field reflecting the sum of the individual birefringences of each segment times their respective lengths. The state of polarization at the output will also fluctuate with time due to environmental conditions and physical changes in the configuration of the optical fiber and network.
Over distances, variations in some of these effects may cancel one another out, so that the resultant polarization mode dispersion at a particular node or receiver is relatively small. Conversely, because these effects are non-uniform, the polarization mode dispersion at one point along a transmission pathway may differ markedly from that at another point, and may also shift significantly over time. A given signal may also encounter different polarization mode dispersion effects when traversing alternate routes, so that the resultant dispersion evident in a signal at one location may depend upon the sum of uncorrected dispersion-causing effects to which that signal was subjected over a longer or unique transmission pathway. Polarization mode dispersion may also be introduced or varied somewhat randomly by the addition or deletion of optically-functioning components in a transmission pathway, such as when operations like amplification, wavelength-division multiplexing, regeneration, or switching are performed.
One approach towards compensating for polarization mode dispersion is specialized single-mode fibers which have polarization mode attenuating or maintaining properties created by intentionally-asymmetric cores. In single-polarization fiber, one polarization mode is transmitted normally, whereas the orthogonal polarization mode is subject to three or four orders of magnitude greater attenuation, effectively stripping that mode and leaving the first for signal transmission. In polarization-maintaining fiber, input light is split into two orthogonal modes along a core having an induced stress or asymmetry which defines an maintains different refractive indices (but similar attenuation values) for each polarization. The two polarization modes may travel at different speeds due to the relative refractive indices, but the light energy does not shift between polarization modes. Polarized light may be aligned with and transmitted along one axis of the polarization-maintaining fiber, in which case a single polarization mode is detected at the receiver. Alternately, both polarization modes may be transmitted, but only one filtered at the detector and used for communication signal transmission.
The use of single-polarization and polarization-maintaining fibers introduces certain limitations and drawbacks into the optical transmission system, such as the need to fabricate a more complex optical fiber geometry, or the need for more specialized transmitter and receiver components capable of aligning or detecting light signals at preferred polarization orientations. The polarization orientations must also be preserved or modified uniformly at various junctions along the optical pathway, such as splices or connections, polarization-dependent optical components, and so forth.
Another approach to the issue of polarization mode dispersion is a class of devices referred to as polarization mode dispersion compensators (or xe2x80x9cPMD compensatorsxe2x80x9d). These devices may be inserted into an optical pathway to detect and correct polarization mode dispersion at a given location (such as immediately before an amplifier, router, or receiver module), and may be periodically adjusted to increase or decrease the level of correction. The device may be adjusted depending upon the degree to which a relatively-constant polarization mode dispersion drifts over time, or may monitor and correct for near instantaneous shifts in polarization mode dispersion.
Though various designs and configurations for polarization mode dispersion compensators have been proposed, conceptually they may generally be regarded as having several common characteristics or operations. First, the two polarization modes must be split or differentiated from one another so that the relative time or phase differential (or xe2x80x9cdifferential group delayxe2x80x9d) between the two modes can be accurately measured. A polarization transformer may also be used in this step to align each polarization mode with a fixed reference or axis. Second, a delay must be introduced into the pathway of the faster or leading polarization mode to counterbalance the measured differential. This variable differential time delay line may have fast and slow axes aligned with the polarization axes induced by the polarization transformer, and the delay line generally imposes a higher degree of delay along at least one axis compared with standard transmission fiber. Third, the polarization modes will be recombined for further transmission or signal processing (if the actual signal was split and measured, as opposed to tapping off a portion of the signal to be measured independently). Finally, some type of forward or backward feedback loop is established to control and periodically adjust the delay.
The process of measuring the differential between the two polarization modes and feeding that information back or forward to the component which responsively introduces the corrective delay into the system can be relatively complex. For example, various statistical methods and predictive modeling schemes have been proposed to accommodate certain system configurations or work effectively with different PMD compensator designs.
Various designs for generating the corrective delay have also been proposed. For example, one approach is to provide a plurality of slightly-differing spans of optical fiber connected via optical switches to the path of the faster polarization mode component of the signal. Each fiber span introduces a predetermined delay based upon its corresponding path length or a physical property of the optical fiber over that span. Given a measured differential, the PMD compensator would switch the faster polarization mode component of the split signal through one or more of these delay fiber spans, so that the total accumulated corrective delay equaled the measured differential (when considering the slower polarization mode component simultaneously traversing a fixed reference span). For a simplified example using delay spans having lengths which provide an increasing series of 0.5 picosecond (ps) delays relative to a fixed reference span, a measured 1.5 ps differential could be corrected by optically routing the faster polarization mode component sequentially through a 1.0 ps delay and a 0.5 ps delay using optical switches (while the slower polarization mode component traverses the fixed reference span) before recombining the two polarization mode components of the signal.
While the operation of such a differential delay component is conceptually simple, it will be readily appreciated that the design requires a plurality of parallel delay spans and optical switches, therefore increasing its complexity for manufacturing, assembly, and control systems. The individual components such as the optical switches might introduce unique variations into the system, therefore requiring tuning or adjustment prior to operation, as well as correcting for thermal or other instabilities which would affect the temporal delay as conditions change or the device ages. In addition, the range of total differentials to be accommodated and the number of discrete corrective delays or combinations to be introduced in some applications may require greater numbers of components and significantly increase the complexity of the device. The optical switches will likely be one limiting factor in the speed at which the PMD compensator can act upon instantaneous PMD variations, and possibly the lower time limit for the fastest corrective delay to be introduced at any one time.
Accordingly, the present invention is an in-line variable differential delay device which may be used for applications such as polarization mode dispersion compensation. Briefly described, in an exemplary embodiment the variable delay device utilizes a plurality of polarization rotators and delay elements aligned in series and alternating with one another along a single transmission pathway. The polarization rotators may be selectively actuated to map the fast polarization mode component of an incoming optical signal to the slow axis of the adjacent delay element, thus imposing an incremental delay on the fast polarization mode component of the optical signal. One or more of the polarization rotators may be actuated to implicate a desired pattern of delay elements (each having a corresponding incremental delay value) in order to achieve a total differential delay value for the device.
One aspect of the present invention is a variable delay component utilized to correct polarization mode dispersion in an optical signal having a fast polarization mode component, a slow polarization mode component, and an initial polarization mode differential, wherein the variable delay component comprises a plurality of delay elements each having a fast axis and a slow axis capable of transmitting a selected polarization mode, and a plurality of polarization selectors optically coupled to the delay elements (in series and alternating with one another) such that the optical signal traverses each of the plurality of polarization selectors and each of the plurality of delay elements as the optical signal traverses the variable delay component. A first polarization selector is actuated to translate the fast polarization mode component of the optical signal to the slow axis of one or more subsequent delay elements so as to impose an incremental delay on the fast polarization mode component of the optical signal. The sum of the incremental delays imposed by the plurality of delay elements on the fast polarization mode component of the optical signal generally corrects for the initial polarization mode differential.
In another aspect, the invention comprises a delay component including a delay element having a fast axis and a slow axis each capable of transmitting a selected polarization mode, and a polarization rotator optically coupled in series with the delay element, the polarization rotator being selectively actuated to rotate the fast polarization mode component of the optical signal to the slow axis of the delay element so as to impose an incremental delay on the fast polarization mode component of the optical signal, or the polarization rotator being selectively actuated to not rotate the fast polarization mode component of the optical signal to the slow axis of the delay element so as not to impose an incremental delay on the fast polarization mode component of the optical signal, such that the incremental delay imposed by the delay element on the fast polarization mode component of the optical signal generally corrects for at least a portion of the initial polarization mode differential.
In a further aspect, the present invention constitutes a method for compensating for polarization mode dispersion in a optical signal having a fast polarization mode component, a slow polarization mode component, and an initial polarization mode differential therebetween comprising steps of providing a plurality of delay elements each having a fast axis and a slow axis capable of transmitting a selected polarization mode, providing a plurality of polarization selectors disposed in series and alternating with the delay elements, and selectively actuating one or more of the polarization selectors in a predetermined pattern to translate the fast polarization mode component of the optical signal to the slow axis of one or more of the plurality of delay elements so as to impose one or more of the incremental delays on the fast polarization mode component of the optical signal, such that the sum of the one or more incremental delays imposed by the plurality of delay elements on the fast polarization mode component of the optical signal generally corrects for the initial polarization mode differential.
Another aspect of the present invention is characterized in a polarization mode dispersion compensator utilized to correct polarization mode dispersion in an optical signal having a fast polarization mode component, a slow polarization mode component, and an initial polarization mode differential therebetween, the polarization mode compensator including a device for discriminating between the fast polarization mode component and the slow polarization mode and for measuring the initial polarization mode differential, a variable delay device for imposing a selected delay on the fast polarization mode component of the optical signal, and a control device for adjusting the variable delay device in response to the initial polarization mode differential measured, wherein the improvement is a variable delay device in which a plurality of delay elements each have a fast axis and a slow axis and an incremental delay, and a plurality of polarization rotators disposed in series alternating with the plurality of delay elements, such that one or more of the polarization rotators may be actuated in a predetermined pattern to selectively rotate the fast polarization mode component of the optical signal to the slow axis of one or more of the plurality of delay elements so as to impose one or more of the incremental delays on the fast polarization mode component of the optical signal, such that the sum of the one or more incremental delays imposed by the plurality of delay elements on the fast polarization mode component of the optical signal generally corrects for the initial polarization mode differential.
Additional aspects, features, and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.